The invention relates to superconducting magnetic coils.
Superconductors may be used to fabricate superconducting magnetic coils such as solenoids, multipole magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the superconductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.
Typical superconducting materials include niobium-titanium, niobium-tin, and also copper oxide ceramics such as members of the rare-earth-copper-oxide family (i.e., YBCO), the thallium-barium-calcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide family (i.e., HgBCCO), and the bismuth-strontium-calcium-copper-oxide family (with or without lead substitutes, i.e.,(Bi,Pb)2Sr2Ca2Cu3O10, Bi2Sr2Ca2Cu3O10 (BSCCO (2223)), which perform particularly well because their superconductivity and corresponding high current density characteristics are achieved at relatively high temperatures (Tc≈108xc2x0 K.)
In fabricating certain superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape which allows the conductor to be bent around the diameter of a core. For example, high temperature superconductor (HTS) is often fabricated as a thin tape in which multi-filament composite superconductor including individual superconducting filaments extends substantially the length of the multi-filament composite conductor and are surrounded by a matrix-forming material, which is typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer (not shown).
One approach for winding a magnetic coil with superconducting tape is known as pancake winding, in which the superconductor tape is wound one turn on top of a preceding turn thereby forming a plane of turns perpendicular to the axis of the coil. In applications where a series of pancake coils are to be used to form a coil, the pancake coils can be wound as double pancakes.
In some applications, a superconducting magnetic coil assembly using pancake coils (whether single or double) may include several coils, coaxially disposed along the length of the coil assembly. The individual coils are interconnected using short lengths of superconducting wire or ribbon made from the superconducting materials of the type described above, for example, copper oxide ceramic.
One example of this arrangement is described in U.S. Pat. No. 5,581,220, assigned to the assignee of the present invention and incorporated herein by reference, which describes a variable-profile (e.g., stair-stepped) superconducting magnetic coil with a varying radial cross section. Another example of a superconducting coil having a stacked arrangement of pancake coils is the xe2x80x9csaddle-shapedxe2x80x9d coil, which is oval-shaped or rectangular with rounded corners.
The variable-profile and saddle-shaped coils are advantageously used in applications where the superconducting magnetic coil is required to conform to or be positioned within an annular region of an assembly, such as a rotating electric machine.
The invention features a superconducting coil having a conical or tapered profile.
In one aspect of the invention, the superconducting coil includes a superconductor tape wound concentrically about and disposed along an axis of the coil to provide a plurality of concentric turns defining an opening having a dimension which gradually decreases, in the direction along the axis, from a first end to a second end of the coil. Each turn of the superconductor tape has a broad surface maintained substantially parallel to the axis of the coil.
The decreasing dimension opening defined by the winding configuration of the coil provides a coil having a tapered profile. The advantages of a tapered superconducting coil having this arrangement are numerous. For example, the tapered superconducting coil is well-suited for use in applications where the coil is to be positioned in annularly-shaped volumes, such as those commonly found in rotating electric machines (e.g., motors or generators.) In general, the tapered arrangement eliminates stepped profiles, common with other stacked arrangements. In particular, the tapered superconducting coil requires relatively fewer stacked individual coils to fill annularly-shaped volumes. This is in contrast to other superconducting coil assemblies, which require stacking of many more thin, individual coils to fill an annularly-shaped volume. Moreover, reducing the number of individual coils, in turn, reduces the number of electrical connections between the individual coils, thereby increasing the overall performance and reliability of a coil assembly using tapered coils.
In addition, the superconductor tape of the present invention is wound with its broad surface maintained substantially parallel to the axis of the coil (as well as to adjacent turns.) This feature is particularly advantageous when the tape is formed of less flexible, brittle materials, such as ceramic-based high temperature superconducting materials.
Furthermore, the tapered configuration provides better critical current (Ic) retention characteristics and allows for better coil grading.
In another aspect of the invention, a superconducting coil assembly includes a plurality of superconducting coils in a stacked arrangement, each having the characteristics described above. Because the turns of superconductor tape for each coil has its broad surface maintained substantially parallel to the axis of the coil assembly, the individual coils are easily stacked without the need for spacers or wedges. In certain embodiments of this aspect of the invention, the coils are substantially identical, which is particularly advantageous in certain applications (e.g., rotating machines) where the coil assembly is to be placed within a predefined annularly-shaped volume.
In certain stacked arrangements, a top and a bottom coil (e.g., pancakes) at each end of the stack (a first end coil and a second end coil, respectively) are preselected to have a higher critical current retention characteristic than critical current retention characteristic of the coils positioned between the top and bottom coils of the stack. Similarly, groups of coils at the top and bottom ends of the coil assembly may be preselected to have a higher critical current retention characteristic. Positioning top and bottom pancake coils with higher critical current retention characteristics in this manner can significantly lower the power loss of the total coil assembly. The top and bottom pancake coils can be preselected by their intrinsic properties or, alternatively, with small changes in the superconductor tape dimensions.
Embodiments of these aspects of the invention may also include one or more of the following features.
The superconductor tape is wound in a racetrack shape defining a pair of opposing arcuate end sections and a pair of opposing substantially straight side sections.
The superconductor tape includes a multi-filament composite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material. The superconductor tape preferably includes an anisotropic high temperature superconductor, such as (Bi, Pb)2Sr2Ca2Cu3O. In alternative embodiments, the superconductor tape includes a copper oxide ceramic, such as those which are members of the rare-earth-copper-oxide family (i.e., YBCO.)
In certain embodiments, the superconductor tape includes a pair of superconductor layers and at least one mechanical reinforcing layer. The pair of superconductor layers are disposed between a pair of mechanical reinforcing layers, each reinforcing layer including stainless steel.
The superconducting coil is in the form of a pancake coil, such as a double pancake coil.
In one embodiment, the superconductor tape is wound to provide a linearly tapered inner surface of the coil extending along the axis of the coil. In an alternative embodiment, the superconductor tape is wound to provide a curved inner surface of the coil extending along the axis of the coil. In this embodiment, the curved inner surface of the wound superconductor tape is cylindrically-shaped along the opposing substantially straight side sections and spherically-shaped along the opposing arcuate end sections.
In another aspect of the invention, a method of providing a superconducting coil includes the following steps. A superconductor tape is wound about an axis of the coil to provide concentric turns defining an opening having an inner dimension with a broad surface of the superconductor tape maintained substantially parallel to the axis of the coil. The tape is wound so that the opening gradually decreases from a first end to a second end in the direction along the axis.
Embodiments of this aspect of the invention includes one or more of the following features.
The superconducting coil is wound using a mandrel having surfaces which define the taper. For example, the surfaces may be linearly tapered or curved (e.g., cylindrically-shaped or spherically-shaped.) In an alternative approach, a pair of heated plates are used to apply heat and pressure to mold the superconducting coil into a tapered profile.
Other advantages and features will become apparent from the following description and the claims.